Method for plugging an aperture with a heat recoverable plug

ABSTRACT

A device for plugging or sealing apertures comprising a heat recoverable dished disc which has the capability to increase its radial dimension upon being heated to a temperature above the transition temperature of the metal. Sealing is accomplished by selecting a heat recoverable dished disc having a radial dimension smaller than that of the aperture to be sealed, but which is capable of increasing its radial dimension to a magnitude greater than that of the aperture. The disc is inserted into the aperture and exposed to a temperature higher than its transition temperature whereupon the disc attempts to return to its original substantially planar configuration thereby increasing in radial dimension and sealing the aperture.

This is a division, of application Ser. No. 320,930, filed Jan. 4, 1973.

BACKGROUND OF THE INVENTION

Many types of closure means are known for sealing or plugging aperturesin metal and other structures, e.g., tanks, drums, etc. The mostcommonly used are threaded screw-type means and force-fitting plugs. Theefficiency of such closure means is directly related to the precisionand tolerances used in sizing the mating surfaces. When the requirementson such devices are highly demanding, e.g., when it is desired to sealagainst the leakage or pressurized gaseous materials, extreme care mustbe taken in preparing the closure means and, often, in preparing theaperture itself as by threading or dressing the seating surfaces whichwill be contacted by a force-fitting plug.

Among the several types of force-fitting closure means presently in useare included so-called "expansion plugs." Expansion plugs are typicallycup-shaped or disc-shaped. In either case the plug is seated by rammingit into place with a rod or similar means. In the case of the cup-typeexpansion plug, a plug which is slightly larger in radial dimension thanthe aperture is selected and such sealing as is obtained results fromthe spring effect of the resilient wall of the cup on the walls of theaperture. Substantial care must be exercised in ramming such plugs,because it is essential that the line of direction of the ramming forcenot to skew to the axis of the cup or else the plug will twist duringseating. Such twisting would, of course, preclude achievement of aneffective seal. In the case of the disc-type expansion plug, a deformeddisc which has one surface which is generally convex and another surfacewhich is generally concave, and which may be of a generally dishedconfiguration, is inserted into an aperture having a radial dimensionslightly larger than the radial dimension of the disc. Upon ramming, thedisc flexes and such sealing as is obtained results from the radialforces which the flexed disc is capable of exerting. The recommendedmanner of setting such discs is to so size them such that when they arerammed they are capable of turning inside out, i.e., the concave surfacebecomes convex and vice versa, and then spring back to an almost flatposition. In order to obtain such action, it is recognized that closetolerances must be achieved. In this regard, it is recommended that theangle between the cylindrical sides and the bottom of the counterbore inthe aperture should be 95° to 97°. Such discs are subject to manydifficulties, including the need to exercise very substantial care inpreparing the seating surfaces in the aperture as by counterboring, thetendency of such plugs to self-release under high pressures,particularly when the plug is installed such that a concave surfacefaces the interior of the chamber to be sealed. Lastly, both types ofexpansion plugs are subject to the deficiency that a ramming force whichhas a jarring effect on the chamber to be sealed must be used. Suchforces are often highly undesirable, since they cause vibrations whichmay loosen other fittings, disturb the contents of the chamber, or, inextreme cases, create risk of explosion.

SUMMARY OF THE INVENTION

The present invention relates to heat recoverable plugs which can berapidly and reliably installed in apertures to be sealed, together witha method of using the same. The heat recoverable plug is manufacturedfrom a metal having the capability of changing shape when it passes fromthe martensitic phase into the austenitic phase. Preferably, the plug isprepared by deforming a thin disc of metal having the capability ofhaving heat recoverability imparted to it into a dish shape, i.e., suchthat one surface is convex and the other concave. When exposed totemperatures above the transition temperature, i.e., that temperature atwhich there is transition from martensitic to austenitic phase, the discattempts to return to its original planar configuration and, as it triesto flatten out, it expands in diameter. The heat recoverable plug is soselected that the radial dimension of the aperture is less than theradial dimension of the disc in its flat configuration. Thus, the wallsof the aperture prevent full recovery to flatness and tremendous forcesare developed causing circumferential penetration of the disc edge intothe sidewall of the aperture. This provides positive, metal-to-metalengagement and achieves a very effective seal.

While the preferred embodiment of the present invention contemplates theuse of a planar disc which has been deformed into a dish shape, it is tobe understood that many other configurations of plug may be used,depending on the configuration of the aperture to be sealed and thespecial effects, if any, which may be desired. While the plug in theusual case will be solid, it may, if desired, be perforated or otherwiseiregular in configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a plug of the present inventionafter installation in an aperture.

FIG. 2 is a perspective view of a heat-recoverable plug afterdeformation, but before installation.

FIG. 3 shows a disc and forming dies prior to deformation.

FIG. 4 shows the disc and dies of FIG. 3 after deformation.

FIG. 5 shows a deformed disc having another type of positioning deviceremovably attached thereto.

FIG. 6 is a cross-sectional view of a plug of the present inventionafter installation on a shelf in an aperture.

DESCRIPTION OF THE INVENTION

The foregoing summary is only a brief description of the presentinvention. In order to understand the invention more completely, certainproperties of the metals from which the plugs of the present inventionmay be made must be discussed in more detail. It is to be understoodthat the following discussion, and all of the theories and principlesexpressed herein, are set forth to further understanding of theinvention, but, while they are the best information presently availableto the inventor, are not to be considered in any manner restrictive ofthe invention nor does the operability or utility of the inventiondepend on the inventor's belief as to its theory of operation.

As used herein, the term "heat recoverable" means that a material hasbeen deformed from an original, heat-stable configuration to a differentconfiguration in which it will remain until raised above a certaintemperature upon which it will return to its heat-stable configuration.The deformation used to place the material in the heat-unstableconfiguration will be referred to herein as "thermally recoverableplastic deformation." A material which may be so deformed and recoveredis referred to herein as a "material capable of having the property ofheat recoverability imparted thereto." The temperature at which a changein configuration occurs upon heating is referred to herein as the"transition temperature." It should be understood that the transitiontemperature may be a temperature range, and that a hysteresis usuallyoccurs which causes the precise temperature at which a transition takesplace to depend on whether the temperature is rising or falling.Further, the transition temperature is a function of the stress appliedto the material, the temperature rising with increasing stress.

Examples of metallic materials which are capable of having the propertyof heat recoverability imparted thereto are the alloys disclosed in U.S.Pat. Nos. 3,012,882, 3,174,851, and 3,567,523 and Belgium Pat. No.703,649 as well as U.S. Pat. applications Ser. Nos. 51,809 (nowabandoned) and 52,112 (now U.S. Pat. No. 3,753,700), both filed July 2,1970, and assigned to the assignee of the present application, thedisclosures of which are incorporated by reference herein to illuminatethe background of the present invention. However, it is to be understoodthat the present invention is not limited to the use of any particulartype of heat-recoverable metal, but rather comprehends the use of anyheat-recoverable metal, whether now known or discovered in the future.

Generally speaking, metals having transition temperature within therange of + 135° C to - 196° C are satisfactory for use in thisinvention. The preferred embodiment of the present invention comprisespreparation of a planar disc from a nickel titanium alloy containing47.1% nickel, 49.4% titanium, and 3.5% iron. This composition results ina material with a transition temperature of approximately -125° C. Thedisc is reduced to a temperature below its transition temperature byimmersing it in Liquid Nitrogen which exhibits a stable temperature of -196° C. While held at this reduced temperature, the disc is subjected tosufficient force, e.g., in a press, to cause thermally recoverableplastic deformation to take place whereby the disc is placed in aheat-unstable dished configuration. The deformed disc is then held at atemperature below its transition temperature, e.g., by continuedimmersion in liquid nitrogen, until it is ready for use.

If the specific disc having a diameter of 1.02 inches and a thickness of0.04 inches is to be used to seal a hole having an aperture of 1.00inches, it is subjected to sufficient deformation to cause its radialdimension to be reduced to less than a diameter of 1.00 inches, e.g., to0.98 inches. When it is desired to use the heat-unstable plug, it isremoved from its low temperature environment and placed in the apertureto be plugged. As the plug warms to room temperature, the materialpasses through its transition temperature and undergoes transition fromthe martensitic to the austenitic phase. As this occurs, the discattempts to revert to its original flat configuration and expands indiameter. As shown in FIG. 1, after installation and recovery, the plug1 retains a dished configuration because the walls of aperture 2 inplate 3 have a diameter less than the original flat diameter of theplug. Tremendous forces are generated during recovery causingpenetration of the disc edge 4 into the sidewall of the aperture. Thisprovides a highly effective seal. When subjected to a helium leak test,it has been found that such a plug permits no detectable leakage whenpressurized with helium (mass spectrometer sensitivity; 1 × 10⁻ ¹⁰scc/sec). In addition, no hydraulic leakage occurred when the plug washeld at incremental pressures up to 1,000 psi and in burst tests ventedpressure at 1100 to 1200 psi. It is important to recognize that theseresults were achieved with no critical tolerances or surface finishesbecause of the great magnitude of the forces exerted by the plug duringthermal recovery.

Preferably, the plug is installed such that the convex surface faces thehigher pressure side of the chamber to be sealed. When installed in thismanner, sealing forces will increase with the application of pressurecomplementing the sealing forces generated by the thermal recovery ofthe disc.

For purposes of clarity, plug 1 is shown in perspective in FIG. 2 as itappears prior to installation in an aperture.

FIGS. 3 and 4 illustrate one means for assuring that the axis of theplug is substantially perpendicular to the plane of the wall in whichthe aperture to be sealed is located. FIG. 3 illustrates a flat disc 4to which a male forming die 5 is removably attached, e.g., by glue. Thedisc with attached die is reduced to below its transition temperatureand forming pressure is applied to die 5 by means of female die 5a todeform disc 4 as shown in FIG. 4. Lip 6 on die 5 will then serve tolocate disc 4 in the aperture to be sealed. After the dished disc 4 isinserted into the aperture and allowed to warm to above its transitiontemperature whereby the aperture is sealed, die 5 is removed by simplypulling with enough force to break the glue bond with the disc. Die 5may be fabricated from plastic, metal, or other suitable material. Theconfiguration of die 5 will vary according to the original shape or disc4 and the configuration of the deformation which it is desired toimpart.

Another embodiment of this invention is shown in FIG. 5 wherein a "hat"8 is removably attached to disc 7 after deformation. Lip 9 on hat 8functions to position disc 7 in the same manner as lip 6 of die 5.

Still further, as shown in FIG. 6, the wall of the aperture to be sealedmay be counterbored or countersunk to provide a lip or shelf 11 on the"inner" or "bottom " portion of the wall to aid in positioning the plugof this invention. The lip or shelf 11 described above may be continuousor discontinuous. When such a shelf is employed, the male die would notrequire any positioning lip such as those shown in FIGS. 3-5. The outeredge 10 of the disc 1 may be provided with grooves, ridges, teeth, orother means to maximize its effectiveness upon recovery of the disc.

It will be readily apparent to those skilled in the art that theforegoing invention may be modified as to configuration of plugs, choiceof heat-recoverable metal, and otherwise without departing from theconcept of the present invention. In this regard, it is pointed out thatthe original configuration of the plug need not be that of a planardisc. However, in the typical situation where a round aperture is to besealed, it is believed that the use of a planar disc which is deformedto a dish-shaped configuration is the most convenient and effectiveembodiment of this invention.

What is claimed is:
 1. A method of sealing an aperture comprisinginserting into said aperture a heat-recoverable plug, said plug havingbeen deformed from a heat-stable configuration it possesses in itsaustenitic state to a heat-recoverable configuration which it maintainswhile in its martensitic state, said plug having a transverse dimensionsmaller in its heat-recoverable condition than in its heat-stablecondition and warming said plug to a temperature above a transitiontemperature at which it reverts from its martensitic state to itsaustenitic state whereby the transverse dimension of said plug increasesand the circumference of said plug is brought into sealing contact withthe walls of said aperture.
 2. The method of claim 1 wherein said plughas been deformed from a substantially flat configuration in itsheat-stable condition to a dished configuration in its heat-recoverablecondition and the transverse dimension between the walls of saidaperture is smaller than the transverse dimension which would bepossessed by said plug if said plug recovered to a flat configuration.3. A method according to claim 1 wherein said plug is an alloy oftitanium and nickel.
 4. A method according to claim 2 wherein theaperture is round and the plug, in its heat-stable state, is a round,substantially flat disc.